Chemistry

Grahams Law of diffusion, Worked Example and Importance

len Alfred Ajibola - Mon, 29th April, 2019 @ 16:19: PM

Topics in Chemistry

Chemistry Scheme of Work, SS1, First Term Chemistry Scheme of Work, SS1, Second Term Chemistry Scheme of Work, SS1, Third Term Compounds in Chemistry: Characteristics of Compounds Types of Mixture: Homogenous and Heterogeneous Mixtures Mixtures: Characteristics of Mixtures Physical Properties of Matter explained with Examples Chemical Properties of Matter explained with Examples Differences between Physical and Chemical Change Ionic Theory and Electrolysis Physical and Chemical Properties of Acids States of Matter: Characteristics of Solids Liquid State of Matter: Characteristics of Liquids Gaseous State of Matter: Characteristics of Gases Molar Mass, Worked Examples and Differences with Molecular Mass Elements: Characteristics of Elements Salts and Types of Salt in Chemistry Mixtures and Separation of Mixtures Mixtures and Separation of Mixtures - part 2 Mixtures and Separation of Mixtures - part 3


Academic Questions in Chemistry

Please check out our Test Your Knowledge page to see all Questions and Answers

The Separating Funnel is a technique used in the separation of mixtures. Another name for Separating Funnel is the _____ Funnel.

  • A. Partition
  • B. Separatory
  • C. Division
  • D. Liquid
  • E. Half
  • F. Dividing

2NH4+(aq) + Ag+(aq) -> 2H4AgN

The equation above represents _____ acid.

  • A. Svante Arrhenius'

  • B. Brønsted–Lowry's

  • C. Gilbert Lewis'

  • D. Avogadro's

  • E. Sir Isaac Newton's

  • F. Albert Einstein's

According to Brønsted Thomas Martin Lowry, an acid is defined as _____.

  • A. Any substance that produces hydrogen ion (H+) when dissolved in water

  • B. A substance that accepts electron pair in a chemical reaction

  • C. A substance that gives out hydronium ion (H3O+) when dissolved in water

  • D. A substance that donates a proton in a chemical reaction

  • E. A substance that reacts with a base to form salt and water

  • F. A substance whose hydrogen ions  (H+) are displaced after a reaction with metals higher in the activity, reactivity or electrochemical series

Which of the following is NOT a characteristic of chemical change?

  • A. A new thing is formed
  • B. It cannot be easily separated into it's constituents elements
  • C. A new chemical formula accompanies whatever is formed
  • D. The burning of candle is an example of a chemical change
  • E. Blood is an example of a chemical change
  • F. Iron rust is an example of a chemical change

Which of the following statement is FALSE concerning a physical change _____.

  • A. Substances formed in a physical change have a unique chemical formula and IUPAC name
  • B. It's a change that is easily reversible
  • C. It's a change that can be separated via an appropriate separation technique
  • D. No new substance is formed
  • E. A mixture of salt and water is a physical change
  • F. Physical change can result to a change in color and shapes of substances

The following substances will sublime EXCEPT _____.

  • A. Sodium Cryolite
  • B. Solid Carbon(IV)oxide
  • C. Arsenic
  • D. Sulphur
  • E. Iodine Crystals
  • F. Naphthalene

Which of the following is a state of matter?

  • A. Ions
  • B. Plasma
  • C. Compounds
  • D. Molecules
  • E. Atoms
  • F. None of the above

The ideal way of separating a mixture of blood is through _____.

  • A. Chromatography
  • B. Fractional Distillation
  • C. Centrifugation
  • D. Filtration
  • E. Separating Funnel
  • F. Fractional Crystallization

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Graham’s Law of Diffusion:

Diffusion involves the movement of gaseous, liquid and solid particles from a region of higher concentration to that of lower concentration.

Consider the analogy below:

When a perfume is sprayed in a room, the particles of the perfume will move from a region of higher concentration (region where the perfume was sprayed) to a region of lower concentration (other regions in the room where the perfume wasn’t sprayed). As a result, anyone present in this region of lower concentration will also perceive the scent of the perfume.

This movement of particles from a region of higher concentration to a region of lower concentration is known as diffusion.

The particles undergoing diffusion can be gaseous, liquid or solid; and the movement of such particles must be from a region of higher concentration to a region of lower concentration

Please read on Physical Change, Chemical Change, Elements, Compounds and Mixtures here.

Diffusion is fastest in gaseous particles. This is due to the fact that gaseous particles are able to move freely since the cohesive or binding forces between them are extremely small or negligible.

Please read on the Characteristics of Solids, Liquids and Gases here.

Note: Diffusion can also occur in liquid and solid particles. It is slowest in solid particles.

Diffusion is technically not the same thing as effusion. Diffusion always accompanies effusion; with both happening almost simultaneously.

Effusion is the movement of gaseous particles through a very small opening; and is always accompained by diffusion.

For instance; when an inflated balloon is pierced with a needle, both effusion and diffusion will occur.

When diffusion occurs, it continues until equilibrium is attained. This will imply that; from our previous instance, the scent of the perfume will continue to move from the region of higher concentration to the region of lower concentration until every part of the room becomes equally filled, (assuming all external openings have been shut).

Note: When a gas has a high rate of diffusion, it will take a shorter time for diffusion to occur in such gas; (that is, it will diffuse rapidly).

Conversely, diffusion will take a longer time for gases with lower rate of diffusion; (that is, such gases will diffuse slowly).

The Scottish chemist Thomas Graham (1805-1869) studied the rate at which gases diffuse. He established a law of diffusion (called Graham’s law of diffusion).

Graham’s law of diffusion states that the rate (r) of diffusion in a gas at a given temperature is inversely proportional to the square root of its density or molecular mass (m).

Please read on Molecular Mass and Molar Mass here.

Below is an equation on Graham's Law of Diffusion.

Diffusion Equation - Len Academy

Also Note that: "The vapour density (v.d) of a gas is equal to half its relative molecular mass(r.m.m).

Therefore: r.m.m. = 2 x v. d

Please read on Density and Specific Gravity here.


Worked Example of Graham's Law

30cm3 of a gas with an empirical formula of CH3 diffuses through a porous partition in 45.2s. If 30 cm3 of hydrogen diffused in 11.7s under the same conditions. Calculate:

  1. The vapour density of the CH3 gas
  2. The molecular formula of the gas CH3

(Mass of H2 = 2)

Solution

(i) Vapour density of the CH3 gas

Using the formula derived from the above diagram:

tx/tH= √mx /√mH

t = time, m = molar mass, x = CH3 and H = hydrogen

Time taken for CH3 to diffuse = 45.2s

Time taken for H2 to diffuse = 11.7s

Mass of H2 = 2

Mass of CH3 = ?

Let's substitute these values into the above formula:

45.2/11.7 = √(mx/2)

 2(45.2/11.7)2 = mx

mx = 2 x 14.92 

mx = 29.84g

Mass of CH3 = 29.84g

r.m.m = 2 x v.d

Where r.m.m = Relative Molecular Mass and v.d = Vapour Density

v.d = r.m.m./2

v.d = 29.84/2

v.d = 14.92


(ii) Molecular formula of the gas CH3

xCH3 = 30

15x = 30

x = 2

Therefore, molecular formula = C2H6 which is Ethane

Please read on Emperical and Molecular Formula here.

 

Importance of Graham's law:

  1. It is applied in the separation of gases with different densities.
    Please read on Separation of Mixtures here.
  2. It is applied in the separation of elements with the same atomic number but different mass number (Isotopes).
    Please read on Dalton's Atomic Theory and it's modifications here.
  3. To determine the densities and molecular masses of unknown gases by comparing their rates of diffusion with known gases.

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Alfred Ajibola is a Medical Biochemist, a passionate Academician with over 7 years of experience, a Versatile Writer, a Web Developer, a Cisco Certified Network Associate and a Cisco CyberOps Associate.


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Amazing facts in Chemistry

The only letters that failed to appear on the periodic table are letters:

J     &     Q

Gold and copper are the only two non-silvery colored metals.

Please read some interesting facts about gold here

Copper is the only metal that is naturally antibacterial. This is why you would see some children utilize 'copper' water bottles in schools

Water actually freezes faster when it’s warm than when it’s cold

Most element in their pure state exists physically in different forms. For instance, pure carbon can exist as both diamond and graphite. This phenomenon is called allotropy.

Please read on modification of Dalton's atomic theory here.

If you pour a handful of salt into a full glass of water, the water level will go down rather than overflowing the glass.

Similarly, if you mix half liter of water and half litre of alcohol, the total volume of the liquid will be les than one litre


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